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Re: Using TTL value of response packets on nmap port scans.
From: David Fifield <david () bamsoftware com>
Date: Thu, 12 Apr 2012 21:21:37 -0700

On Fri, Apr 13, 2012 at 07:11:34AM +0300, Otto Airamo wrote:
Hello,

It seems that I was explaining in the details WHAT I would like from Nmap,
but I was missing WHY this is needed.

When port scan is executed over TCP, there is in most cases three relevant
cases that may occur in each port that is scanned.

1)
SYN-ACK packet is received. In most cases this means that port is open in
target host.

2)
No reply. In this case we do not know if target host is listening port. At
least it is known that from this scope* this port is not accessible in
target host. (*With scope I mean that either network firewall or personal
firewall blocks incoming SYN/outgoing RST/ICMP error message. It is
possible that port is open in target host and would answer with SYN-ACK if
packet would come from, for example, same network or different IP address)

Important point here is that WE DO NOT KNOW if port is actually open (from
different scope)!

3) RST packet is received. Typically this means that port is closed in
target host. If RST packet is generated by target host TCP/IP stack
because port is closed, it does not matter what is the scope where we are
performing scan. Result will remain same even if scan would be performed
from same network/using any source IP address. At the same time RST packet
may be generated by firewall located between scanner and target host. If
scanner is not able to tell if RST packet is generated by target host or
device between, result of the scan will fall into same category as case
#2. (We do not know if port is open)

If scanner would be able to detect difference between RST packet
generated by target host TCP/IP stack and other cases, scanner would
have better knowledge of target.

Options are:
a) Target host TCP/IP stack generated RST packet => we know that port is
closed
b) Somebody else than target host TCP/IP stack generated RST packet => we
do not know if port is actually open even we do not have access from this
scope.

Now we have following network setup:

scanner -- FW -- target

Firewall is configured to RST connections to port 22. Some ports are
allowd, mosts of the ports are dropped silently.

Result of nmap could be:
# nmap --reason target --top-ports=10
Interesting ports on target:
PORT     STATE    SERVICE      REASON           TTL
21/tcp   open     ftp          syn-ack          62
22/tcp   refused  ssh          reset (ttl?)     63
23/tcp   closed   telnet       reset            62
25/tcp   filtered smtp         no-response      N/A
80/tcp   open     http         syn-ack          62
110/tcp  filtered pop3         no-response      N/A
139/tcp  filtered netbios-ssn  no-response      N/A
443/tcp  filtered https        no-response      N/A
445/tcp  filtered microsoft-ds no-response      N/A
3389/tcp filtered ms-term-serv no-response      N/A

Thanks for explaining what you mean. The output example especially makes
it clear.

I don't think we would add a TTL column to the output by default. The
information is already in the XML for tools that want to use it. Also,
by convention, states like "closed" are understood to mean that a RST
was received, or whatever else is appropriate for the type of port scan.
The --reason flag is for those who want to see the reason without any
interpretation.

The firewalk script also does what you describe in a thorough manner.
http://nmap.org/nsedoc/scripts/firewalk

David Fifield
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